The prior art screw has a thread with unique orientation. With reference to FIG. 12, an operation for screwing a prior art screw is illustrated. It is illustrated that the screw is screwed into a work piece with a constant orientation. This results in that the screw is tightly clamped by the work piece so that the friction force is larger and thus the screwing work is difficult. When the screw is longer, the difficulty in locking the screw is enhanced.